Microstructural control of NdFeB cast ingots for achieving 50 MGOe sintered magnets

Abstract

The combination of chemical composition and microstructure of cast alloy has been found to be critical to the performance of NdFeB sintered magnets. Maximizing the amount of Nd2Fe14B phase (or minimizing the amount of secondary phase) by reducing the Nd content more closely to the stoichiometric composition appears to be essential for obtaining high BHmax magnets. However, α-Fe precipitation has been found to increase with decreasing Nd content and severely hinders the development of high BHmax magnets. A two-step method, incorporating ingot casting and isothermal annealing, has been developed to minimize the amount of precipitated α-Fe in low Nd content alloys. This method provides a drastic improvement in the Br and BHmax of sintered magnets obtained. By decreasing the Nd content to 13 at. % in the cast alloy, incorporating better particle control during fine milling, and controlling grain growth during sintering; magnets with a Br of more than 14.5 kG and a BHmax of 50 MGOe have been consistently obtained. Furthermore, because of the reduction in the amount of Nd-rich grain boundary phase, a significant improvement in the corrosion resistance of magnets was also observed.